Patent GB-778234-A (GB)
Title: Improvements in or relating to electrical insulation
IPC: H01B3/08
Filed: 1954-10-07
Abstract: 778,234. Flexible laminates ; coated polystyrene foil ; impregnating for electric insulation. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Oct. 7, 1954 [Oct. 15, 1953], No. 28878/54. Drawings to Specification. Class 140. A tape suitable for electric insulation which is to be impregnated with a thermosetting resin comprises a backing of doubly oriented polystyrene, a layer of mica flakes laid thereon, and a liquid resinous binder uniting the mica flakes and the backing. The binder may comprise a solution in an alcohol alone or mixed with esters, such as butyl acetate, or ketones, e.g. a mixture of ethyl alcohol and acetone, of liquid resinous polymers, e.g. of acrylic acid, its α-substituted alkyl, cycloalkyl, phenyl or alkylphenyl derivatives, or alkyl esters of these acids, propylene ; phenyl ethylene (styrene), alkylphenylethylene, or their α-methyl derivatives ; #-pinene ; or polyester resins of saturated or unsaturated mono- or di-carboxylic acids and saturated aliphatic alcohols or glycols or glycerol ; or mixtures of such polymers. Typical polymers for the binder of which numerous examples are mentioned are polymers of styrene, coumarone or indene or copolymers or interpolymers thereof, polymethylmethacrylate of molecular weight 1000, or polyesters produced from fumaric and adipic acids with diethylene or propylene glycols. The backing is first covered with mica flakes and is then passed under a drip pan which applies the binder solution. Before drying the tape to evaporate the solvent, a covering strip of doubly oriented polystyrene may be laid over the applied binder. Coils wound with insulated electrical conductors are wrapped with such a mica tape and are dried at temperatures not above 100 C. with or without application of vacuum. An outer wrapping of glass fibre tape may be applied to resist abrasion in use. The dried coil is vacuum impregnated with a liquid resinous composition which cures to a solid resin without evolving volatile matter and which preferably dissolves the polystyrene backing of the tape so as to allow ready penetration of the impregnating resin. Suitable impregnating compositions are liquid aromatic monomers such as styrene, its α-methyl and p-methyl derivatives, and diallyl phthalate with polyester resins formed by reacting unsaturated or mixtures of saturated and unsaturated dicarboxylic acids with polyhydric alcohols. The polyester resins may be those used as the binder for the mica tape. Instead of glycols epoxides may be reacted with dicarboxylic acids to form the polyester resins while maleic anhydride esterified with castor oil may be used. The curable impregnating resin may also be diallyl phthalate alone, or other resins such as polyepoxide resins comprising glycidyl ethers, and the reaction products of 1 mol. of bisphenol and 1 to 2 mols. of epichlorhydrin. After impregnation, the coil is wrapped with a tape which is impenetrable by the impregnating resin such as cellulose acetate, polyvinyl alcohol, polyvinylidene chloride, polytetrafluoroethylene (PTFE), chloroprene, polyethyleneglycolterephthalate, and polythene. If PTFE tape is used it may be covered with a thinner more elastic polythene tape. The wrapped coil is then pressed to shape in a heated mould and then removed and cured at a higher temperature (120 C.). Specification 670,548 is referred to.